drm/amdgpu/gfx9: drop unnecessary 64-bit fence flag check in KIQ

drm/amdgpu/gfx9: drop unnecessary 64-bit fence flag check in KIQ

Remove the BUG_ON(flags & AMDGPU_FENCE_FLAG_64BIT) assertion from
gfx_v9_0_ring_emit_fence_kiq().  The KIQ hardware supports 64-bit
fence writes; the 32-bit writeback address constraint is an
upper-layer convention, not a hardware limitation.  The check serves
no purpose and should not be present.

Found by code inspection while investigating related BUG_ON
assertions in the GFX and compute ring emission paths.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: John B. Moore <jbmoore61@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>